An inhibitor of plant virus infection was found in the extracts of Mirabilis jalapa L. The inhibitor, a protein designated as MAP (Mirabilis antiviral protein), showed highly potent activity against mechanical transmission of viruses, tobacco mosaic (TMV), cucumber green mottle mosaic, potato Y, turnip mosaic and cucumber mosaic. Almost complete inhibition was achieved when MAP at 0.8μg/ml was applied on the upper surface of leaves of Xanthi nc tobacco 24hr before TMV inoculation, and 50% inhibition when MAP at 10μg/ml was applied on the under surface of leaves. MAP induced systemic resistance to TMV infection in tobacco plants when it was applied on their basal leaves 24hr before TMV inoculation onto the upper ones. MAP showed no inhibition when it was applied 1hr after TMV inoculation. Antiserum against MAP gave positive reaction in the agar gel with leaf extracts of M. jalapa and did not with those of the other Chenopodiales plants such as Basella rubra, Boerhaavia diffusa, Bougainvillea sp., Chenopodium amaranticolor and Phytolacca americana. Concentration of MAP was successfully quantified using enzyme-linked immunosorbent assay. MAP content in the root tissue was exceedingly higher than in the other parts of the plant and that of an yellow-flower trait of M. jalapa was as high as 1.1mg/g fresh weight. M. jalapa seems to be a promising source of antiviral substances for the practical use.